This invention relates, in particular, to antennas to receive signals from Global Positioning System (GPS) satellites for Differential GPS (DGPS) applications and more generally to antennas providing a near-spherical antenna pattern of circular polarization. For present purposes, such an antenna pattern is defined as providing a radiation pattern of circular polarization (e.g., right-hand circular polarization) in all directions, except within a downward cone having an acute internal angle.
Application of the GPS for aircraft approach and landing guidance is subject to various local and other errors limiting accuracy. Implementation of DGPS can provide local corrections to aircraft to improve accuracy of obtainable position information during landings. A DGPS surface (e.g., ground or ship) installation can provide local corrections for errors as may result from ionospheric and tropospheric effects and from satellite clock and ephemeris errors. Of particular significance for DGPS applications is the desirability of use of antennas having the characteristic of a unitary phase center of closely fixed position, to permit highly accurate determinations of phase of received signals and avoid introduction of phase discrepancies.
In surface installations of reference antennas for DGPS applications, reflected multipath signals generally represent the dominant contributor to the total system error. Such signals may be reflected from vertical surfaces and other objects, as well as from ground and water surfaces. In addition to adequate discrimination against ground or water surface reflections, a particular problem is the providing of adequate discrimination against multipath signals reflected from nearby objects and surfaces such as from masts, other antennas, etc., as when an antenna must be positioned on a ship in a crowded and space-limited environment, for example. It is also desirable that antennas for these and other applications be of reasonable size and cost-effective construction.
GPS satellite signals of right-hand circular polarization upon reflection undergo a change to left-hand circular polarization. Thus, in theory an ideal DGPS reference antenna would have an antenna pattern with right-hand circular polarization for all incident directions, that is, it would have a right-hand polarization pattern which was spherical. However, no known antenna design can provide such a spherical pattern and such a pattern is theoretically not realizable.
Objects of the present invention are to provide new and improved antennas, including such antennas usable for DGPS applications and which may provide an antenna pattern with a near-spherical same-hand circular polarization characteristic (with the term same-hand meaning one or the other of right-hand or left-hand circular polarization).